diff --git a/docs/api-reference/expressions.md b/docs/api-reference/expressions.md
index c01dd8fe6..c487ff0ff 100644
--- a/docs/api-reference/expressions.md
+++ b/docs/api-reference/expressions.md
@@ -10,6 +10,7 @@
         - cast
         - diff
         - drop_nulls
+        - fill_null
         - filter
         - is_between
         - is_in
diff --git a/docs/api-reference/series.md b/docs/api-reference/series.md
index 84e66e404..e1171c31f 100644
--- a/docs/api-reference/series.md
+++ b/docs/api-reference/series.md
@@ -11,6 +11,7 @@
         - diff
         - drop_nulls
         - dtype
+        - fill_null
         - filter
         - is_between
         - is_in
diff --git a/narwhals/_pandas_like/expr.py b/narwhals/_pandas_like/expr.py
index 6b002a41d..180fa4da8 100644
--- a/narwhals/_pandas_like/expr.py
+++ b/narwhals/_pandas_like/expr.py
@@ -178,6 +178,9 @@ def is_between(
     def is_null(self) -> Self:
         return register_expression_call(self, "is_null")
 
+    def fill_null(self, value: Any) -> Self:
+        return register_expression_call(self, "fill_null", value)
+
     def is_in(self, other: Any) -> Self:
         return register_expression_call(self, "is_in", other)
 
diff --git a/narwhals/_pandas_like/series.py b/narwhals/_pandas_like/series.py
index 0bbe52657..28747366c 100644
--- a/narwhals/_pandas_like/series.py
+++ b/narwhals/_pandas_like/series.py
@@ -335,6 +335,10 @@ def is_null(self) -> PandasSeries:
         ser = self._series
         return self._from_series(ser.isna())
 
+    def fill_null(self, value: Any) -> PandasSeries:
+        ser = self._series
+        return self._from_series(ser.fillna(value))
+
     def drop_nulls(self) -> PandasSeries:
         ser = self._series
         return self._from_series(ser.dropna())
diff --git a/narwhals/expression.py b/narwhals/expression.py
index 915766f9d..e7100d4e2 100644
--- a/narwhals/expression.py
+++ b/narwhals/expression.py
@@ -604,6 +604,10 @@ def is_null(self) -> Expr:
         """
         Returns a boolean Series indicating which values are null.
 
+        Notes:
+            pandas and Polars handle null values differently. Polars distinguishes
+            between NaN and Null, whereas pandas doesn't.
+
         Examples:
             >>> import pandas as pd
             >>> import polars as pl
@@ -657,6 +661,64 @@ def is_null(self) -> Expr:
         """
         return self.__class__(lambda plx: self._call(plx).is_null())
 
+    def fill_null(self, value: Any) -> Expr:
+        """
+        Fill null values with given value.
+
+        Notes:
+            pandas and Polars handle null values differently. Polars distinguishes
+            between NaN and Null, whereas pandas doesn't.
+
+        Examples:
+            >>> import pandas as pd
+            >>> import polars as pl
+            >>> import narwhals as nw
+            >>> df_pd = pd.DataFrame(
+            ...         {
+            ...             'a': [2, 4, None, 3, 5],
+            ...             'b': [2.0, 4.0, float("nan"), 3.0, 5.0]
+            ...         }
+            ... )
+            >>> df_pl = pl.DataFrame(
+            ...         {
+            ...             'a': [2, 4, None, 3, 5],
+            ...             'b': [2.0, 4.0, float("nan"), 3.0, 5.0]
+            ...         }
+            ... )
+
+            Let's define a dataframe-agnostic function:
+
+            >>> def func(df_any):
+            ...     df = nw.from_native(df_any)
+            ...     df = df.with_columns(nw.col('a', 'b').fill_null(0))
+            ...     return nw.to_native(df)
+
+            We can then pass either pandas or Polars to `func`:
+
+            >>> func(df_pd)
+                 a    b
+            0  2.0  2.0
+            1  4.0  4.0
+            2  0.0  0.0
+            3  3.0  3.0
+            4  5.0  5.0
+
+            >>> func(df_pl)  # nan != null for polars
+            shape: (5, 2)
+            ┌─────┬─────┐
+            │ a   ┆ b   │
+            │ --- ┆ --- │
+            │ i64 ┆ f64 │
+            ╞═════╪═════╡
+            │ 2   ┆ 2.0 │
+            │ 4   ┆ 4.0 │
+            │ 0   ┆ NaN │
+            │ 3   ┆ 3.0 │
+            │ 5   ┆ 5.0 │
+            └─────┴─────┘
+        """
+        return self.__class__(lambda plx: self._call(plx).fill_null(value))
+
     # --- partial reduction ---
     def drop_nulls(self) -> Expr:
         return self.__class__(lambda plx: self._call(plx).drop_nulls())
diff --git a/narwhals/series.py b/narwhals/series.py
index 16d9a9560..ef549c6fc 100644
--- a/narwhals/series.py
+++ b/narwhals/series.py
@@ -196,6 +196,9 @@ def sort(self, *, descending: bool = False) -> Self:
     def is_null(self) -> Self:
         return self._from_series(self._series.is_null())
 
+    def fill_null(self, value: Any) -> Self:
+        return self._from_series(self._series.fill_null(value))
+
     def is_between(
         self, lower_bound: Any, upper_bound: Any, closed: str = "both"
     ) -> Self:
diff --git a/tests/expr/fill_null_test.py b/tests/expr/fill_null_test.py
new file mode 100644
index 000000000..f48815495
--- /dev/null
+++ b/tests/expr/fill_null_test.py
@@ -0,0 +1,32 @@
+from typing import Any
+
+import pandas as pd
+import polars as pl
+import pytest
+
+import narwhals as nw
+from tests.utils import compare_dicts
+
+data = {
+    "a": [0.0, None, 2, 3, 4],
+    "b": [1.0, None, None, 5, 3],
+    "c": [5.0, None, 3, 2, 1],
+}
+
+
+@pytest.mark.parametrize("constructor", [pd.DataFrame, pl.DataFrame])
+def test_over_single(constructor: Any) -> None:
+    df = nw.from_native(constructor(data), eager_only=True)
+    result = df.with_columns(nw.all().fill_null(99))
+    expected = {
+        "a": [0.0, 99, 2, 3, 4],
+        "b": [1.0, 99, 99, 5, 3],
+        "c": [5.0, 99, 3, 2, 1],
+    }
+    compare_dicts(result, expected)
+    result = df.with_columns(
+        a=df["a"].fill_null(99),
+        b=df["b"].fill_null(99),
+        c=df["c"].fill_null(99),
+    )
+    compare_dicts(result, expected)